It is the melody from a piece of Mozart. You’ll see that bars 1-4 (up to the double bar line) is section A. After the double bar line starts a new section for four bars. After that in bar 9 section A is repeated but an octave higher.

With the repeat marks at bar 4 you might write the structure like this:

AABABA – (without the repeat marks it would like this: ABA). This makes the structure Ternary.

A canon is a device used by composers to create a contrapuntal texture. (contrapuntal just means that the melodies work in contrast to each other). The most famouse canon is probably “Row, Row, Row Your Boat”.Composers through the ages have used this device to create textured music and will often apply their own style (or the style of the era). For example, you can hear canons in 18th century Baroque Music as well as 20th century minimalist music.